1. Field of the Invention
This invention relates to optical filters, and more particularly it relates to an electronically tunable optical filter utilizing the electro-optic effect and capable of being programmed to provide a wide variety of filter transmission characteristics.
2. Description of the Prior Art Including Prior Art Statement
One form of optical filter was described by Ivan Solc in an article "Birefringent Chain Filters", Journal of the Optical Society of America, Vol. 55, No. 6 (June 1965), pp. 621-625. In this filter a series of birefringent plates is disposed along the direction of propagation of a light beam between a pair of polarizers having their respective pass directions for polarized light disposed orthogonal to one another. The respective optic axes of successive birefringent plates are oriented at alternating positive and negative small angles of like magnitude with respect to the pass direction of the input polarizer. Light is transmitted through the filter over narrow passbands surrounding wavelengths for which the individual birefringent plates perform as half-wave plates or odd multiple half-wave plates. Since the pass wavelengths are determined by the plate thickness and the birefringence of the plate material, in a filter of the Solc type the pass wavelengths are limited to particular fixed values.
An electronically tunable optical filter has been disclosed by Steven E. Harris in U.S. Pat. No. 3,679,288, issued July 25, 1972, and by Harris and his co-workers in the publications "Acousto-Optic Tunable Filter", Journal of the Optical Society of America, Vol. 59, No. 6 (June 1969), pp. 744-747; "Electronically Tunable Acousto-Optic Filter", Applied Physics Letters, Vol. 15, No. 10 (Nov. 15, 1969), pp. 325-326; and "CaMoO.sub.4 electronically Tunable Optical Filter", Applied Physics Letters, Vol. 17, No. 5 (Sept. 1, 1970), pp. 223-225. In this type of filter, a light wave of one polarization is diffracted into an orthogonal polarization by propagating through a birefringent crystal (disposed between a pair of crossed polarizers) collinearly with an acoustic wave generated in the crystal. This diffraction occurs for a narrow band of light frequencies satisfying the relation that the sum of the momentum vectors of the incident light and acoustic waves equals the momentum vector of the output light wave. By changing the acoustic frequency, the light frequency which is most effectively diffracted into the orthogonal polarization is varied, and the filter passband is changed accordingly.
Since the acousto-optic filter requires a traveling acoustic wave which is lost from the system after one transit through the crystal, power is continually required to generate new acoustic waves of transitory lifetimes. As a result, this type of filter consumes a large amount of power. Moreover, the filter requires an electro-acoustic transducer to convert the electrical control signals into acoustic waves. Since the frequency response of such transducers is limited, not only is a bandwidth limitation imposed on the optical tuning range, but excessive bulk acoustic absorption may occur at higher frequency Fourier components when the transducer is driven at harmonics of its fundamental frequencies.
Another form of optical filter is a multilayer interference filter consisting of alternate layers of materials having high and low indices or refraction. When an electric field is applied across such a filter, a change in the magnitude of the index of refraction occurs in accordance with the electro-optic effect to change the optical path length through the filter and thereby vary the filter passband. Optical filters of this type are disclosed as components of light shutters in U.S. Pat. Nos. 2,960,914 and 3,164,665, both entitled "Electro-Optical Light Shutter". The index of refraction change (hence the passband shift) achievable with multilayer interference optical filters for practical values of applied voltage is sufficiently small to limit the usefulness of such filters to particular applications such as the light shutters of the aforementioned patents. Moreover, such filters cannot be electrically programmed to provide multiple passbands or a single passband of variable bandwidth.
The electro-optic effect also has been utilized to achieve light modulation by changing the magnitude of the indices of refraction of an electro-optic crystal in accordance with a time varying modulating voltage. A light modulator of this type is disclosed in U.S. Pat. No. 4,054,362 entitled "Device for Modulating Light". Although this patent refers to use of the device as a controlled filter, such a filter is not electronically tunable, i.e., its pass or stop wavelengths are not variable in accordance with an applied electrical signal.